Showing 1–44 of 44 results for author: Miranda, R

Scalable Bayesian Optimization for High-Dimensional Coarse-Grained Model Parameterization

Authors: Carlos A. Martins Junior, Daniela A. Damasceno, Keat Yung Hue, Caetano R. Miranda, Erich A. Müller, Rodrigo A. Vargas-Hernández

Abstract: Coarse-grained (CG) force field models are extensively utilised in material simulations due to their scalability. Traditionally, these models are parameterized using hybrid strategies that integrate top-down and bottom-up approaches; however, this combination restricts the capacity to jointly optimize all parameters. While Bayesian Optimization (BO) has been explored as an alternative search strat… ▽ More Coarse-grained (CG) force field models are extensively utilised in material simulations due to their scalability. Traditionally, these models are parameterized using hybrid strategies that integrate top-down and bottom-up approaches; however, this combination restricts the capacity to jointly optimize all parameters. While Bayesian Optimization (BO) has been explored as an alternative search strategy for identifying optimal parameters, its application has traditionally been limited to low-dimensional problems. This has contributed to the perception that BO is unsuitable for more realistic CG models, which often involve a large number of parameters. In this study, we challenge this assumption by successfully extending BO to optimize a high-dimensional CG model. Specifically, we show that a 41-parameter CG model of Pebax-1657, a copolymer composed of alternating polyamide and polyether segments, can be effectively parameterized using BO, resulting in a model that accurately reproduces key physical properties of its atomistic counterpart. Our optimization framework simultaneously targets density, radius of gyration, and glass transition temperature. It achieves convergence in fewer than 600 iterations, resulting in a CG model that shows consistent improvements across all three properties. △ Less

Submitted 27 June, 2025; originally announced June 2025.

Comments: 22 pages, 8 figures, (SI 9 pages, 2 figures, 8 tables)

Influence of carrier density and disorder on the Quantum Hall plateau widths in epitaxial graphene

Authors: Ignacio Figueruelo-Campanero, Yuriko Baba, Alejandro Jimeno-Pozo, Julia García-Pérez, Elvira M. González, Rodolfo Miranda, Francisco Guinea, Enrique Cánovas, Daniel Granados, Pierre Pantaleón, Pablo Burset, Mariela Menghini

Abstract: Since its discovery, graphene has been one of the most prominent 2D materials due to its unique properties and broad range of possible applications. In particular, the half-integer Quantum Hall Effect (HI-QHE) characterized by the quantization of Hall resistivity as a function of applied magnetic field, offers opportunities for advancements in quantum metrology and the understanding of topological… ▽ More Since its discovery, graphene has been one of the most prominent 2D materials due to its unique properties and broad range of possible applications. In particular, the half-integer Quantum Hall Effect (HI-QHE) characterized by the quantization of Hall resistivity as a function of applied magnetic field, offers opportunities for advancements in quantum metrology and the understanding of topological quantum states in this 2D material. While the role of disorder in stabilizing quantum Hall plateaus (QHPs) is widely recognized, the precise interplay between the plateaus width, disorder, mobility and carrier density remains less explored. In this work, we investigate the width of the $ν=6$ QHP in epitaxial graphene Hall bars, focusing on two distinct regions of the device with markedly different electronic mobilities. Depending on the storage conditions, it is possible to modify the carrier density of graphene QHE devices and consequently increase or reduce the mobility. Our experiments reveal mobility variations of up to 200$\%$ from their initial value. In particular, the sample storage time and ambient conditions cause also noticeable changes in the positions and extension of the QHPs. Our results show that the QHP extension for $ν=6$ differs significantly between the two regions, influenced by both mobility and disorder, rather than solely by carrier density. Transport simulations based on the Landauer-Büttiker formalism with Anderson disorder in a scaled model reveal the critical role of impurities in shaping graphene transport properties defining the extension of the QHPs. This study provides valuable insights into the interplay between mobility, disorder, and quantum transport in graphene systems. △ Less

Submitted 15 January, 2025; v1 submitted 13 January, 2025; originally announced January 2025.

Comments: 9 pages, 4 figures, regular research article

Simulation of bright and dark diffuse multiple scattering lines in high-flux synchrotron X-ray experiments

Authors: M. B. Estradiote, A. G. A. Nisbet, R. F. S. Penacchio, M. A. R. Miranda, G. A. Calligaris, S. L. Morelhao

Abstract: We present a theoretical framework for understanding diffuse multiple scattering (DMS) in single crystals, focusing on diffuse scattering-Bragg (DS-Bragg) channels. These channels, when probed with high-flux, low-divergent monochromatic synchrotron X-rays, provide well-defined visualizations of Kossel lines. Our main contribution lies in modeling the intensity distribution along these lines by con… ▽ More We present a theoretical framework for understanding diffuse multiple scattering (DMS) in single crystals, focusing on diffuse scattering-Bragg (DS-Bragg) channels. These channels, when probed with high-flux, low-divergent monochromatic synchrotron X-rays, provide well-defined visualizations of Kossel lines. Our main contribution lies in modeling the intensity distribution along these lines by considering DS around individual reciprocal lattice nodes. The model incorporates contributions from both general DS and mosaicity, elucidating their connection to second-order scattering events. This comprehensive approach advances our understanding of DMS phenomena, enabling their use as probes for complex material behavior, particularly under extreme conditions. △ Less

Submitted 21 December, 2024; v1 submitted 17 June, 2024; originally announced June 2024.

Comments: 8 pages, 8 figures, previously presented at MRS fall meeting 2023 (10.13140/RG.2.2.29836.08325)

Unveiling the inter-layer interaction in a 1H/1T TaS$_2$ van de Waals heterostructure

Authors: Cosme G. Ayani, M. Bosnar, F. Calleja, Andrés Pinar Solé, O. Stetsovych, Iván M. Ibarburu, Clara Rebanal, Manuela Garnica, Rodolfo Miranda, M. M. Otrokov, M. Ondráček, Pavel Jelínek, A. Arnau, Amadeo L. Vázquez de Parga

Abstract: This study delves into the intriguing properties of 1H/1T-TaS$_2$ van der Waals heterostructure, focusing on the transparency of the 1H layer to the Charge Density Wave of the underlying 1T layer. Despite the sizable interlayer separation and metallic nature of the 1H layer, positive bias voltages result in a pronounced superposition of the 1T charge density wave structure on the 1H layer. The con… ▽ More This study delves into the intriguing properties of 1H/1T-TaS$_2$ van der Waals heterostructure, focusing on the transparency of the 1H layer to the Charge Density Wave of the underlying 1T layer. Despite the sizable interlayer separation and metallic nature of the 1H layer, positive bias voltages result in a pronounced superposition of the 1T charge density wave structure on the 1H layer. The conventional explanation relying on tunneling effects proves insufficient. Through a comprehensive investigation combining lowtemperature scanning tunneling microscopy, scanning tunneling spectroscopy, non-contact atomic force microscopy, and firstprinciples calculations, we propose an alternative interpretation. The transparency effect arises from a weak yet substantial electronic coupling between the 1H and 1T layers, challenging prior understanding of the system. Our results highlight the critical role played by interlayer electronic interactions in van der Waals heterostructures to determine the final ground states of the systems. △ Less

Submitted 26 February, 2024; originally announced February 2024.

Directional picoantenna behavior of tunnel junctions in the presence of atomic-scale defects

Authors: David Mateos, Oscar Jover, Miguel Varea, Koen Lauwaet, Daniel Granados, Rodolfo Miranda, Antonio I. Fernandez-Dominguez, Alberto Martin-Jimenez, Roberto Otero

Abstract: Plasmonic nanoantennas, metallodielectric structures with engineered size and shape, have attracted much attention lately as they make the control of the directionality and temporal characteristics of light emitted by fluorophores possible. Nanoantennas exploit light-matter interactions mediated by Localized Surface Plasmon Resonances and, so far, have been demonstrated using metallic nanoparticle… ▽ More Plasmonic nanoantennas, metallodielectric structures with engineered size and shape, have attracted much attention lately as they make the control of the directionality and temporal characteristics of light emitted by fluorophores possible. Nanoantennas exploit light-matter interactions mediated by Localized Surface Plasmon Resonances and, so far, have been demonstrated using metallic nanoparticles or other metallic nanostructures. Plasmonic picocavities, i.e., plasmonic cavities with mode volumes below 1 cubic nanometer, could act as antennas to mediate light-matter interaction even more efficiently than their nanoscale counterparts due to their extreme field confinement, but the directionality on their emission is difficult to control. In this work, we show that the plasmonic picocavity formed between the tip of a Scanning Tunnelling Microscope and a metal surface with a monoatomic step shows directional emission profiles and, thus, can be considered as a realization of a picoantenna. Comparison with electromagnetic calculations demonstrates that the observed directionality arises from light emission tilting of the picocavity plasmons. Our results, thus, pave the way to exploiting picoantennas as an efficient way to control light-matter interaction at the nanoscale. △ Less

Submitted 2 February, 2024; originally announced February 2024.

Engineering a Spin-Orbit Bandgap in Graphene-Tellurium Heterostructures

Authors: B. Muñiz Cano, D. Pacilè, M. G. Cuxart, A. Amiri, F. Calleja, M. Pisarra, A. Sindona, F. Martín, E. Salagre, P. Segovia, E. G. Michel, A. L. Vázquez de Parga, R. Miranda, J. Camarero, M. Garnica, M. A. Valbuena

Abstract: Intensive research has focused on harnessing the potential of graphene for electronic, optoelectronic, and spintronic devices by generating a bandgap at the Dirac point and enhancing the spin-orbit interaction in the graphene layer. Proximity to heavy p elements is a promising approach; however, their interaction in graphene heterostructures has not been as intensively studied as that of ferromagn… ▽ More Intensive research has focused on harnessing the potential of graphene for electronic, optoelectronic, and spintronic devices by generating a bandgap at the Dirac point and enhancing the spin-orbit interaction in the graphene layer. Proximity to heavy p elements is a promising approach; however, their interaction in graphene heterostructures has not been as intensively studied as that of ferromagnetic, noble, or heavy d metals, neither as interlayers nor as substrates. In this study, the effective intercalation of Te atoms in a graphene on Ir(111) heterostructure is achieved. Combining techniques such as low energy electron diffraction and scanning tunneling microscopy, the structural evolution of the system as a function of the Te coverage is elucidated, uncovering up to two distinct phases. The presented angle-resolved photoemission spectroscopy analysis reveals the emergence of a bandgap of about 240 meV in the Dirac cone at room temperature, which preserves its characteristic linear dispersion. Furthermore, a pronounced n-doping effect induced by Te in the heterostructure is also observed, and remarkably the possibility of tuning the Dirac point energy towards the Fermi level by reducing the Te coverage while maintaining the open bandgap is demonstrated. Spin-resolved measurements unveil a non-planar chiral spin texture with significant splitting values for both in-plane and out-of-plane spin components. These experimental findings are consistent with the development of a quantum spin Hall phase, where a Te-enhanced intrinsic spin orbit coupling in graphene surpasses the Rashba one and promotes the opening of the spin-orbit bandgap. △ Less

Submitted 28 November, 2023; originally announced November 2023.

Comments: 9 pages, 4 figures

Revisiting greenhouse gases adsorption in carbon nanostructures: advances through a combined first-principles and molecular simulation approach

Authors: Henrique Musseli Cezar, Teresa Duarte Lanna, Daniela Andrade Damasceno, Alexsandro Kirch, Caetano Rodrigues Miranda

Abstract: Carbon nanostructures are promising materials to improve the performance of current gas separation membrane technologies. From the molecular modeling perspective, an accurate description of the interfacial interactions is mandatory to understand the gas selectivity in the context of greenhouse gases applications. Most of the molecular dynamics simulations studies considered available force fields… ▽ More Carbon nanostructures are promising materials to improve the performance of current gas separation membrane technologies. From the molecular modeling perspective, an accurate description of the interfacial interactions is mandatory to understand the gas selectivity in the context of greenhouse gases applications. Most of the molecular dynamics simulations studies considered available force fields with the standard Lorentz-Berthelot (LB) mixing rules to describe the interaction among carbon dioxide (CO2), methane (CH4) and carbon structures. We performed a systematic study in which we showed the LB underestimates the fluid/solid interaction energies compared to the density functional theory (DFT) calculation results. To improve the classical description, we propose a new parametrization for the cross-terms of the Lenard-Jones (LJ) potential by fitting DFT forces and energies. The effects of the new parametrization on the gases adsorption within single-walled carbon nanotubes (SWCNTs) with varying diameters, are investigated with Grand Canonical Monte Carlo simulations. We observed considerable differences in the CO2 and CH4 density within SWCNTs compared to those obtained with the standard approach. Our study highlights the importance of going beyond the traditional LB mixing rules in studies involving solid/fluid interfaces of confined systems. The revised mixing terms enhanced fluid/carbon interface description with excellent transferability ranging from SWCNTs to graphene. △ Less

Submitted 27 November, 2023; v1 submitted 21 July, 2023; originally announced July 2023.

Mechanical and adsorption properties of greenhouse gases filled carbon nanotubes

Authors: Daniela Andrade Damasceno, Henrique Musseli Cezar, Teresa Duarte Lanna, Alexsandro Kirch, Caetano Rodrigues Miranda

Abstract: We investigate the mechanical and adsorption properties of single-walled carbon nanotubes (SWCNTs) filled with greenhouse gases through Grand Canonical Monte Carlo (GCMC) and Molecular Dynamics (MD) simulations using a recently developed parameterization for the cross-terms of the Lenard-Jones (LJ) potential. Carbon nanotubes interact strongly with CO$_2$ compared to CH$_4$, resulting in a CO$_2$-… ▽ More We investigate the mechanical and adsorption properties of single-walled carbon nanotubes (SWCNTs) filled with greenhouse gases through Grand Canonical Monte Carlo (GCMC) and Molecular Dynamics (MD) simulations using a recently developed parameterization for the cross-terms of the Lenard-Jones (LJ) potential. Carbon nanotubes interact strongly with CO$_2$ compared to CH$_4$, resulting in a CO$_2$-rich composition inside the nanotubes, with the proportion of CO$_2$ decreasing as the diameter of the nanotubes increases. Contrarily, the smallest nanotubes showed a more even balance between CO$_2$ and CH$_4$ due to gas solidification. The gas does not affect the mechanical response of the nanotubes under tension, but under compression, it presents a complex relationship with the loading direction, nanotube's diameters, chirality, and to a minor extent, the gas composition. Filled zigzag nanotubes showed to be more stable in the presence of fillers, giving the best mechanical performance compared to the filled armchairs. The study confirms carbon nanotubes as effective means of separating CO$_2$ from CH$_4$, presenting good mechanical stability. △ Less

Submitted 21 July, 2023; originally announced July 2023.

Effects of van der Waals interaction on the N$_2$ adsorption on carbon nanotubes: proposal of new force field parameters

Authors: Carlos Alberto Martins Junior, Henrique Musseli Cezar, Daniela Andrade Damasceno, Caetano Rodrigues Miranda

Abstract: The separation of carbon dioxide CO$_2$ from nitrogen gas (N$_2$), the main component of flue gas, has become an emerging action to mitigate climate change. Feasible and efficient approaches to exploring the separation properties of materials are molecular dynamics (MD) and Monte Carlo (MC) simulations. In these approaches, a careful choice of force fields is required to avoid unrealistic predicti… ▽ More The separation of carbon dioxide CO$_2$ from nitrogen gas (N$_2$), the main component of flue gas, has become an emerging action to mitigate climate change. Feasible and efficient approaches to exploring the separation properties of materials are molecular dynamics (MD) and Monte Carlo (MC) simulations. In these approaches, a careful choice of force fields is required to avoid unrealistic predictions of thermodynamic properties. However, most studies use Lorentz-Berthelot combining rules (LB) to obtain the interaction between different species, an approximation that could not capture the essence of interfacial interactions. In this context, we verified how accurate LB is in describing the interaction of N$_2$ molecules and carbon nanostructures by comparing the interaction energies from LB with those from density functional theory (DFT) calculations. We selected carbon nanomaterials because they are considered promising materials to perform N$_2$/CO$_2$ separation. The results show that the LB underestimates the interaction energies and affects the prediction of fundamental properties of solid-fluid interfacial interactions. To overcome this limitation, we parametrized a Lennard-Jones potential using energies and forces from DFT, obtained through the van der Waals functional KBM. The proposed potential show good transferability and agreement to ab-initio calculations. Grand Canonical Monte Carlo simulations were performed to verify the effects of employing LB in predicting the amount of nitrogen gas adsorbed inside different CNTs. LB predicts a lower density inside them. Moreover, our results suggest that LB leads to a different characterization of the adsorption properties of carbon nanotubes, by changing significantly the adsorption isotherm. △ Less

Submitted 19 July, 2023; originally announced July 2023.

Water adsorption in ultrathin silica nanotubes

Authors: Henrique Musseli Cezar, Caetano Rodrigues Miranda

Abstract: Silica (SiO$_2$) nanotubes (NTs) are used in a wide range of applications that go from sensors to nanofluidics. Currently, these NTs can be grown with diameters as small as 3 nm, with walls 1.5 nm thick. Recent experimental advances combined with first-principles calculations suggest that silica NTs could be obtained from a single silica sheet. In this work, we explore the water adsorption in such… ▽ More Silica (SiO$_2$) nanotubes (NTs) are used in a wide range of applications that go from sensors to nanofluidics. Currently, these NTs can be grown with diameters as small as 3 nm, with walls 1.5 nm thick. Recent experimental advances combined with first-principles calculations suggest that silica NTs could be obtained from a single silica sheet. In this work, we explore the water adsorption in such ultrathin silica NTs using molecular simulation and first-principles calculations. Combining molecular dynamics and density functional theory calculations we obtain putative structures for NTs formed by 10, 12, and 15-membered SiO$_2$ rings. Water adsorption isotherms for these NTs are obtained using Grand Canonical Monte Carlo simulations. Computing the accessible cross-section area ($A_\text{free}$) for the NTs, we were able to understand how this property correlates with condensation pressures. We found that $A_\text{free}$ does not necessarily grow with the NT size and that the higher the confinement (smaller $A_\text{free}$), the larger the condensation pressure. △ Less

Submitted 19 July, 2023; originally announced July 2023.

Experimental demonstration of a magnetically induced warping transition in a topological insulator mediated by rare-earth surface dopants

Authors: Beatriz Muñiz Cano, Yago Ferreiros, Pierre A. Pantaleón, Ji Dai, Massimo Tallarida, Adriana I. Figueroa, Vera Marinova, Kevin García Díez, Aitor Mugarza, Sergio O. Valenzuela, Rodolfo Miranda, Julio Camarero, Francisco Guinea, Jose Angel Silva-Guillén, Miguel A. Valbuena

Abstract: Magnetic topological insulators (MTI) constitute a novel class of materials where the topologically protected band structure coexists with long-range ferromagnetic order, which can lead to the breaking of time-reversal symmetry (TRS), introducing a bandgap in the Dirac cone-shaped topological surface state (TSS). The gap opening in MITs has been predicted to be accompanied by a distortion in the T… ▽ More Magnetic topological insulators (MTI) constitute a novel class of materials where the topologically protected band structure coexists with long-range ferromagnetic order, which can lead to the breaking of time-reversal symmetry (TRS), introducing a bandgap in the Dirac cone-shaped topological surface state (TSS). The gap opening in MITs has been predicted to be accompanied by a distortion in the TSS, evolving its warped shape from hexagonal to trigonal. In this work, we demonstrate such a transition by means of angle-resolved photoemission spectroscopy after the deposition of low concentrations of magnetic rare earths, namely Er and Dy, on the ternary three-dimensional prototypical topological insulator Bi$_2$Se$_2$Te. Signatures of the gap opening occurring as a consequence of the TRS breaking have also been observed, whose existence is supported by the observation of the aforementioned transition. Moreover, increasing the Er coverage results in a tunable p-type doping of the TSS. As a consequence, the Fermi level (E$_{\textrm{F}}$) of our Bi$_2$Se$_2$Te crystals can be gradually tuned towards the TSS Dirac point, and therefore to the magnetically induced bandgap; thus fulfilling two of the necessary prerequisites for the realization of the quantum anomalous Hall effect (QAHE) in this system. The experimental results are rationalized by a theoretical model where a magnetic Zeeman out-of-plane term is introduced in the hamiltonian governing the TSS band dispersion. Our results offer new strategies to control magnetic interactions with TSSs based on a simple approach and open up viable routes for the realization of the QAHE. △ Less

Submitted 3 February, 2023; originally announced February 2023.

Metastable polymorphic phases in monolayer TaTe2

Authors: Iolanda Di Bernardo, Joan Ripoll-Sau, Fabian Calleja, Cosme G. Ayani, Rodolfo Miranda, Jose Angel Silva-Guillen, Enric Canadell, Manuela Garnica, Amadeo L. Vazquez de Parga

Abstract: Polymorphic phases and collective phenomena - such as charge density waves (CDWs) - in transition metal dichalcogenides (TMDs) dictate the physical and electronic properties of the material. Most TMDs naturally occur in a single given phase, but the fine-tuning of growth conditions via methods like molecular beam epitaxy (MBE) allows to unlock otherwise inaccessible polymorphic structures. Explori… ▽ More Polymorphic phases and collective phenomena - such as charge density waves (CDWs) - in transition metal dichalcogenides (TMDs) dictate the physical and electronic properties of the material. Most TMDs naturally occur in a single given phase, but the fine-tuning of growth conditions via methods like molecular beam epitaxy (MBE) allows to unlock otherwise inaccessible polymorphic structures. Exploring and understanding the morphological and electronic properties of new phases of TMDs is an essential step to enable their exploitation in technological applications. Here, we use scanning tunneling microscopy to map MBE-grown monolayer TaTe2. We report the first observation of the 1H polymorphic phase, coexisting with the 1T, and demonstrate that their relative coverage can be controlled by adjusting synthesis parameters. Several super-periodic structures, compatible with CDWs, are observed to coexist on the 1T phase. Finally, we provide theoretical insight on the delicate balance between Te...Te and Ta-Ta interactions that dictates the stability of the different phases. Our findings demonstrate that TaTe2 is an ideal platform to investigate competing interactions, and indicate that accurate tuning of growth conditions is key to accessing metastable states in TMDs. △ Less

Submitted 16 January, 2023; originally announced January 2023.

Journal ref: Small 19, 2300262 (2023)

Selectively addressing plasmonic modes and excitonic states in a nanocavity hosting a quantum emitter

Authors: Alberto Martín-Jiménez, Óscar Jover, Koen Lauwaet, Daniel Granados, Rodolfo Miranda, Roberto Otero

Abstract: Understanding and controlling the interaction between the excitonic states of a quantum emitter and the plasmonic modes of a nanocavity is one of the most relevant current scientific challenges, key for the development of many applications, from quantum information processing devices to polaritonic catalysts. In this paper we demonstrate that the tunnel electroluminescence of C60 nanocrystals encl… ▽ More Understanding and controlling the interaction between the excitonic states of a quantum emitter and the plasmonic modes of a nanocavity is one of the most relevant current scientific challenges, key for the development of many applications, from quantum information processing devices to polaritonic catalysts. In this paper we demonstrate that the tunnel electroluminescence of C60 nanocrystals enclosed in the plasmonic nanocavity between a metallic surface and the tip of a Scanning Tunnelling Microscope, and isolated from the metal surface by a thin NaCl film, can be switched from a broad emission spectrum, revealing the plasmonic modes of the cavity, to a narrow band emission, displaying only the excitonic states of the C60 molecules by changing the bias voltage applied to the junction. Plasmonic emission is found in the same voltage region in which the rate of inelastic tunnel transitions is large and, thus, vanishes for large voltages. Excitonic emission, on the other hand, dominates the spectra in the high-voltage region in which the inelastic rate is low, demonstrating that the excitons cannot be created by an inelastic tunnel process. These results point towards new possible mechanisms to explain the tunnel electroluminescence of quantum emitters and offer new avenues to develop electrically tuneable nanoscale light sources. △ Less

Submitted 30 June, 2022; originally announced July 2022.

Rashba-like spin textures in Graphene promoted by ferromagnet-mediated Electronic-Hybridization with heavy metal

Authors: Beatriz Muñiz Cano, Adrían Gudín, Jaime Sánchez-Barriga, Oliver J. Clark, Alberto Anadón, Jose Manuel Díez, Pablo Olleros-Rodríguez, Fernando Ajejas, Iciar Arnay, Matteo Jugovac, Julien Rault, Patrick Le Févre, François Bertran, Donya Mazhjoo, Gustav Bihlmayer, Stefan Blügel, Rodolfo Miranda, Julio Camarero, Miguel Angel Valbuena, Paolo Perna

Abstract: Epitaxial graphene/ferromagnetic metal (Gr/FM) heterostructures deposited onto heavy metals (HM) have been proposed for the realization of novel spintronic devices because of their perpendicular magnetic anisotropy and sizeable Dzyaloshinskii-Moriya interaction (DMI), allowing for both enhanced thermal stability and stabilization of chiral spin textures. However, establishing routes towards this g… ▽ More Epitaxial graphene/ferromagnetic metal (Gr/FM) heterostructures deposited onto heavy metals (HM) have been proposed for the realization of novel spintronic devices because of their perpendicular magnetic anisotropy and sizeable Dzyaloshinskii-Moriya interaction (DMI), allowing for both enhanced thermal stability and stabilization of chiral spin textures. However, establishing routes towards this goal requires the fundamental understanding of the microscopic origin of their unusual properties. Here, we elucidate the nature of the induced spin-orbit coupling (SOC) at Gr/Co interfaces on Ir. Through spin- and angle-resolved photoemission along with density functional theory, we show that the interaction of the HM with the C atomic layer via hybridization with the FM is the source of strong SOC in the Gr layer. Furthermore, our studies on ultrathin Co films underneath Gr reveal an energy splitting of $\sim$\,100 meV (negligible) for in-plane (out-of-plane) spin polarized Gr $π$ bands, consistent with a Rashba-SOC at the Gr/Co interface, which is either the fingerprint or the origin of the DMI. This mechanism vanishes at large Co thicknesses, where neither in-plane nor out-of-plane spin-orbit splitting is observed, indicating that Gr $π$ states are electronically decoupled from the HM. The present findings are important for future applications of Gr-based heterostructures in spintronic devices. △ Less

Submitted 1 May, 2023; v1 submitted 9 June, 2022; originally announced June 2022.

Comments: 11 pages, 7 figures

Switchable molecular functionalization of an STM tip: from a Yu-Shiba-Rusinov tip to a Kondo tip

Authors: Cosme. G. Ayani, Fabian Calleja, Ivan M. Ibarburu, Pablo Casado, Nana K. M. Nazriq, Toyo. K. Yamada, Manuela. Garnica, Amadeo L. Vázquez de Parga, Rodolfo Miranda

Abstract: In this work we fabricate and characterize a functionalized superconducting (SC) Nb tip of a scanning tunnelling microscope (STM). The tip is functionalized with a Tetracyanoquinodimethane molecule (TCNQ) that accepts charge from the tip and develops a magnetic moment. As a consequence, in scanning tunnelling spectroscopy (STS), sharp, bias symmetric sub-gap states identified as Yu-Shiba-Rusinov (… ▽ More In this work we fabricate and characterize a functionalized superconducting (SC) Nb tip of a scanning tunnelling microscope (STM). The tip is functionalized with a Tetracyanoquinodimethane molecule (TCNQ) that accepts charge from the tip and develops a magnetic moment. As a consequence, in scanning tunnelling spectroscopy (STS), sharp, bias symmetric sub-gap states identified as Yu-Shiba-Rusinov (YSR) bound states appear against the featureless density of states of a metallic gr/Ir(111) sample. Although the coupling regime of the magnetic impurity with the SC tip depends on the initial absorption configuration of the molecule, the interaction strength between the superconducting tip and the charged TCNQ molecule can be reversibly controlled by tuning the tip-sample distance. The controlled transition from one coupling regime to the other allows us to verify the relation between the energy scales of the two competing many-body effects for the functionalized tip. Quenching the SC state of the Nb tip with a magnetic field switches abruptly from a tip dominated by the YSR bound states to a Kondo tip. △ Less

Submitted 24 May, 2022; originally announced June 2022.

Probing the phase transition to a coherent 2D Kondo lattice

Authors: Cosme G. Ayani, Michele Pisarra, Iván M. Ibarburu, Manuela Garnica, Rodolfo Miranda, Fabián Calleja, Fernando Martín, Amadeo L. Vázquez de Parga

Abstract: Strongly correlated materials exhibit exotic electronic states arising from the strong correlation between electrons. Dimensionality provides a tuning knob because thinning down to atomic thickness reduces screening effects and enhances electron correlations. In this work, a 2D Kondo lattice has been created by stacking a layer of 1T-TaS2 on a 2H-TaS2 crystal, which are bound by weak van der Waals… ▽ More Strongly correlated materials exhibit exotic electronic states arising from the strong correlation between electrons. Dimensionality provides a tuning knob because thinning down to atomic thickness reduces screening effects and enhances electron correlations. In this work, a 2D Kondo lattice has been created by stacking a layer of 1T-TaS2 on a 2H-TaS2 crystal, which are bound by weak van der Waals interactions. By using high-resolution scanning tunnelling spectroscopy and density functional theory, we unambiguously demonstrate the formation of a 2D Kondo lattice from an ensemble of independent Kondo impurities present at higher temperatures. The possibility to create 2D Kondo lattices in van der Waals heterostructures paves the way for the exploration of unconventional metallic, magnetic and superconducting states not present in more standard correlated materials. △ Less

Submitted 22 November, 2023; v1 submitted 23 May, 2022; originally announced May 2022.

Native point defects and their implications for the Dirac point gap at MnBi$_2$Te$_4$(0001)

Authors: M. Garnica, M. M. Otrokov, P. Casado Aguilar, I. I. Klimovskikh, D. Estyunin, Z. S. Aliev, I. R. Amiraslanov, N. A. Abdullayev, V. N. Zverev, M. B. Babanly, N. T. Mamedov, A. M. Shikin, A. Arnau, A. L. Vázquez de Parga, E. V. Chulkov, R. Miranda

Abstract: The Dirac point gap at the surface of the antiferromagnetic topological insulator MnBi$_2$Te$_4$ is a highly debated issue. While the early photoemission measurements reported on large gaps in agreement with theoretical predictions, other experiments found vanishingly small splitting of the MnBi$_2$Te$_4$ Dirac cone. Here, we study the crystalline and electronic structure of MnBi$_2$Te$_4$(0001) u… ▽ More The Dirac point gap at the surface of the antiferromagnetic topological insulator MnBi$_2$Te$_4$ is a highly debated issue. While the early photoemission measurements reported on large gaps in agreement with theoretical predictions, other experiments found vanishingly small splitting of the MnBi$_2$Te$_4$ Dirac cone. Here, we study the crystalline and electronic structure of MnBi$_2$Te$_4$(0001) using scanning tunneling microscopy/spectroscopy (STM/S), micro($μ$)-laser angle resolved photoemission spectroscopy (ARPES), and density functional theory (DFT) calculations. Our topographic STM images clearly reveal features corresponding to point defects in the surface Te and subsurface Bi layers that we identify with the aid of STM simulations as Bi$_\text{Te}$ antisites (Bi atoms at the Te sites) and Mn$_\text{Bi}$ substitutions (Mn atoms at the Bi sites), respectively. X-ray diffraction (XRD) experiments further evidence the presence of cation (Mn-Bi) intermixing. Altogether, this affects the distribution of the Mn atoms, which, inevitably, leads to a deviation of the MnBi$_2$Te$_4$ magnetic structure from that predicted for the ideal crystal structure. Our transport measurements suggest that the degree of this deviation varies from sample to sample. Consistently, the ARPES/STS experiments reveal that the Dirac point gap of the topological surface state is different for different samples/sample cleavages. Our DFT surface electronic structure calculations show that, due to the predominant localization of the topological surface state near the Bi layers, Mn$_\text{Bi}$ defects can cause a strong reduction of the MnBi$_2$Te$_4$ Dirac point gap, given the recently proved antiparallel alignment of the Mn$_\text{Bi}$ moments with respect to those of the Mn layer. Our results provide a key to puzzle out the MnBi$_2$Te$_4$ Dirac point gap mystery. △ Less

Submitted 3 September, 2021; originally announced September 2021.

Quantum Brain Networks: a Perspective

Authors: E. R. Miranda, S. Venkatesh, C. Hernani-Morales, L. Lamata, J. D. Martín-Guerrero, E. Solano

Abstract: We propose Quantum Brain Networks (QBraiNs) as a new interdisciplinary field integrating knowledge and methods from neurotechnology, artificial intelligence, and quantum computing. The objective is to develop an enhanced connectivity between the human brain and quantum computers for a variety of disruptive applications. We foresee the emergence of hybrid classical-quantum networks of wetware and h… ▽ More We propose Quantum Brain Networks (QBraiNs) as a new interdisciplinary field integrating knowledge and methods from neurotechnology, artificial intelligence, and quantum computing. The objective is to develop an enhanced connectivity between the human brain and quantum computers for a variety of disruptive applications. We foresee the emergence of hybrid classical-quantum networks of wetware and hardware nodes, mediated by machine learning techniques and brain-machine interfaces. QBraiNs will harness and transform in unprecedented ways arts, science, technologies, and entrepreneurship, in particular activities related to medicine, Internet of humans, intelligent devices, sensorial experience, gaming, Internet of things, crypto trading, and business. △ Less

Submitted 23 June, 2021; originally announced June 2021.

Comments: Four pages and 1 Figure

Journal ref: E. R. Miranda et al. Electronics 11, 1528 (2022)

Magnetic ordering phase transition and abnormal brittleness in dilute Fe-Mn solid solution

Authors: Wei Liu, Yunfeng Liang, Xiangyan Li, Yichun Xu, Yange Zhang, Wenliang Li, Q. F. Fang, Caetano R. Miranda, Chuan-Lu Yang, C. S. Liu, Xuebang Wu

Abstract: Experiments showed that solute Mn in bcc iron is in antiferromagnetic (AFM) coupling with iron neighbours below 2 at.% Mn, but is in ferromagnetic (FM) coupling at higher concentrations. Surprisingly, although Mn is an important alloying element in high-strength steels, it induces brittleness just at around 2 at.% Mn and higher concentrations. However, the mechanisms for the magnetic ordering phas… ▽ More Experiments showed that solute Mn in bcc iron is in antiferromagnetic (AFM) coupling with iron neighbours below 2 at.% Mn, but is in ferromagnetic (FM) coupling at higher concentrations. Surprisingly, although Mn is an important alloying element in high-strength steels, it induces brittleness just at around 2 at.% Mn and higher concentrations. However, the mechanisms for the magnetic ordering phase transition and the abnormal brittleness remain unclear. Based on magnetism-constrained/unconstrained calculations and ab initio molecular dynamics simulations within density functional theory, we show that while the AFM phase prevails at low Mn contents, the FM phase becomes dominant at 1.85 at.% Mn and elevated temperatures. Our results suggest that the AFM-FM phase transition with increasing Mn concentration can be ascribed to the thermal effect. Furthermore, we find that the brittleness of the Fe-Mn alloys at intermediate Mn content might be related to the stress variations within the grains accompanying the local magnetic ordering changes. △ Less

Submitted 8 June, 2024; v1 submitted 25 May, 2021; originally announced May 2021.

Comments: 31 pages, 4 figures

Electronic temperature and two-electron processes in overbias plasmonic emission from tunnel junctions

Authors: Alberto Martín-Jiménez, Koen Lauwaet, Óscar Jover, Daniel Granados, Andrés Arnau, Vyacheslav M. Silkin, Rodolfo Miranda, Roberto Otero

Abstract: The accurate determination of electronic temperatures in metallic nanostructures is essential for many technological applications, like plasmon-enhanced catalysis or lithographic nanofabrication procedures. In this Letter we demonstrate that the electronic temperature can be accurately measured by the shape of the tunnel electroluminescence emission edge in tunnel plasmonic nanocavities, which fol… ▽ More The accurate determination of electronic temperatures in metallic nanostructures is essential for many technological applications, like plasmon-enhanced catalysis or lithographic nanofabrication procedures. In this Letter we demonstrate that the electronic temperature can be accurately measured by the shape of the tunnel electroluminescence emission edge in tunnel plasmonic nanocavities, which follows a universal thermal distribution with the bias voltage as the chemical potential of the photon population. A significant deviation between electronic and lattice temperatures is found below 30 K for tunnel currents larger than 15 nA. This deviation is rationalized as the result of a two-electron process in which the second electron excites plasmon modes with an energy distribution that reflects the higher temperature following the first tunneling event. These results dispel a long-standing controversy on the nature of overbias emission in tunnel junctions and adds a new method for the determination of electronic temperatures and quasiparticle dynamics. △ Less

Submitted 22 February, 2021; originally announced February 2021.

Origin of the Large Perpendicular Magnetic Anisotropy in Nanometer-thick Epitaxial Graphene/Co/Heavy Metal Heterostructures

Authors: M. Blanco-Rey, P. Perna, A. Gudin, J. M. Diez, A. Anadon Leticia de Melo Costa, Manuel Valvidares, Pierluigi Gargiani, Alejandra Guedeja-Marron, Mariona Cabero, M. Varela, C. Garcia-Fernandez, M. M. Otrokov, J. Camarero, R. Miranda, A. Arnau, J. I. Cerda

Abstract: A combination of theoretical modelling and experiments reveals the origin of the large perpendicular magnetic anisotropy (PMA) that appears in nanometer-thick epitaxial Co films intercalated between graphene (Gr) and a heavy metal (HM) substrate, as a function of the Co thickness. High quality epitaxial Gr/Co\n/HM(111) (HM=Pt,Ir) heterostructures are grown by intercalation below graphene, which ac… ▽ More A combination of theoretical modelling and experiments reveals the origin of the large perpendicular magnetic anisotropy (PMA) that appears in nanometer-thick epitaxial Co films intercalated between graphene (Gr) and a heavy metal (HM) substrate, as a function of the Co thickness. High quality epitaxial Gr/Co\n/HM(111) (HM=Pt,Ir) heterostructures are grown by intercalation below graphene, which acts as a surfactant that kinetically stabilizes the pseudomorphic growth of highly perfect Co face-centered tetragonal ($fct$) films, with a reduced number of stacking faults as the only structural defect observable by high resolution scanning transmission electron microscopy (HR-STEM). Magneto-optic Kerr effect (MOKE) measurements show that such heterostructures present PMA up to large Co critical thicknesses of about 4~nm (20~ML) and 2~nm (10~ML) for Pt and Ir substrates, respectively, while X-ray magnetic circular dichroism (XMCD) measurements show an inverse power law of the anistropy of the orbital moment with Co thickness, reflecting its interfacial nature, that changes sign at about the same critical values. First principles calculations show that, regardless of the presence of graphene, ideal Co $fct$ films on HM buffers do not sustain PMAs beyond around 6~MLs due to the in-plane contribution of the inner bulk-like Co layers. The large experimental critical thicknesses sustaining PMA can only be retrieved by the inclusion of structural defects that promote a local $hcp$ stacking such as twin boundaries or stacking faults. Remarkably, a layer resolved analysis of the orbital momentum anisotropy reproduces its interfacial nature, and reveals that the Gr/Co interface contribution is comparable to that of the Co/Pt(Ir). △ Less

Submitted 11 December, 2020; originally announced December 2020.

Comments: 25 pages, 4 figures, 1 table

Tailoring π-conjugation and vibrational modes to steer on-surface synthesis of pentalene-bridged ladder polymers

Authors: Bruno de la Torre, Adam Matěj, Ana Sánchez-Grande, Borja Cirera, Benjamin Mallada, Eider Rodríguez-Sánchez, José Santos, Jesús I. Mendieta-Moreno, Shayan Edalatmanesh, Koen Lauwaet, Michal Otyepka, Miroslav Medveď, Álvaro Buendía, Rodolfo Miranda, Nazario Martín, Pavel Jelínek, David Écija

Abstract: The development of synthetic strategies to engineer π-conjugated polymers is of paramount importance in modern chemistry and materials science. Here we introduce a theoretical and experimental synthetic paradigm based on the search for specific vibrational modes through an appropriate tailoring of the π-conjugation of the precursors, in order to increase the attempt frequency of a chemical reactio… ▽ More The development of synthetic strategies to engineer π-conjugated polymers is of paramount importance in modern chemistry and materials science. Here we introduce a theoretical and experimental synthetic paradigm based on the search for specific vibrational modes through an appropriate tailoring of the π-conjugation of the precursors, in order to increase the attempt frequency of a chemical reaction. First, we on-surface design a 1D π-conjugated polymer with specific π-topology, which is based on bisanthene monomers linked by cumulene bridges that tune specific vibrational modes. In a second step, upon further annealing, such vibrational modes steer the two-fold cyclization reaction between adjacent bisanthene moieties, which gives rise to a long and free-defect pentalene-bridged conjugated ladder polymer featuring a low band gap. In addition, high resolution atomic force microscopy allows us to identify by atomistic insights the resonant form of the polymer, thus confirming the validity of the Glidewell and Lloyd's rules for aromaticity. This on-surface synthetic strategy may stimulate exploiting previously precluded reactions towards novel pi-conjugated polymers with specific structures and properties. △ Less

Submitted 18 April, 2020; v1 submitted 10 April, 2020; originally announced April 2020.

Direct determination of Spin-Orbit torque by using dc current-voltage characteristics

Authors: R. Guerrero, A. Anadon, A. Gudin, J. M. Diez, P. Olleros-Rodriguez, M. Muñoz, R. Miranda, J. Camarero, P. Perna

Abstract: Spin polarized currents are employed to efficiently manipulate the magnetization of ferromagnetic ultrathin films by exerting a torque on it. If the spin currents are generated by means of the spin-orbit interaction between a ferromagnetic and a non-magnetic layer, the effect is known as spin-orbit torque (SOT), and is quantified by measuring the effective fields produced by a charge current injec… ▽ More Spin polarized currents are employed to efficiently manipulate the magnetization of ferromagnetic ultrathin films by exerting a torque on it. If the spin currents are generated by means of the spin-orbit interaction between a ferromagnetic and a non-magnetic layer, the effect is known as spin-orbit torque (SOT), and is quantified by measuring the effective fields produced by a charge current injected into the device. In this work, we present a new experimental technique to quantify directly the SOT based on the measurement of non-linearities of the dc current-voltage (IV) characteristics in Hall bar devices employing a simple instrumentation. Through the analysis of the IV curves, the technique provides directly the linearity of the effective fields with current, the detection of the current range in which the thermal effects can be relevant, the appearance of misalignments artefacts when the symmetry relations of SOT are not fulfilled, and the conditions for the validity of the single domain approximations, which are not considered in switching current and second harmonic generation state-of-the-art experiments. We have studied the SOT induced antidamping and field-like torques in Ta/Co/Pt asymmetric stacks with perpendicular magnetic anisotropy. △ Less

Submitted 6 April, 2020; originally announced April 2020.

Experimental evidence of spin-orbit torque from metallic interfaces

Authors: A. Anadón, R. Guerrero, J. A. Jover-Galtier, A. Gudín, J. M. Díez, P. Olleros-Rodríguez, R. Miranda, J. Camarero, P. Perna

Abstract: Spin currents can modify the magnetic state of ferromagnetic ultrathin films through spin-orbit torque. They may be generated by means of spin-orbit interaction by either bulk or interfacial phenomena. Electrical transport measurements reveal a six-fold increase of the spin-orbit torque accompanied by a drastic reduction of the spin Hall magnetoresistance upon the introduction of a Cu interlayer i… ▽ More Spin currents can modify the magnetic state of ferromagnetic ultrathin films through spin-orbit torque. They may be generated by means of spin-orbit interaction by either bulk or interfacial phenomena. Electrical transport measurements reveal a six-fold increase of the spin-orbit torque accompanied by a drastic reduction of the spin Hall magnetoresistance upon the introduction of a Cu interlayer in a Pt/Cu/Co/Pt structure with perpendicular magnetic anisotropy. We analyze the dependence of the spin Hall magnetoresistance with the thickness of the interlayer in the frame of a drift diffusion model that provides information on the expected spin currents and spin accumulations in the system. The results demonstrate that the major responsible of both effects is spin memory loss at the interface. The enhancement of the spin-orbit torque when introducing an interlayer opens the possibility to design more effient spintronic devices based on materials that are cheap and abundant such as copper. △ Less

Submitted 17 March, 2020; originally announced March 2020.

Comments: 12 pages, 5 figures

Journal ref: ACS Appl. Nano Mater. 2021, 4, 1, 487-492

Thermally Activated Processes for Ferromagnet Intercalation in Graphene-Heavy Metal Interfaces

Authors: F. Ajejas, A. Anadón, A. Gudin, J. M. Diez, C. G. Ayani, P. Olleros, L. de Melo Costa, C. Navío, A. Gutierrez, F. Calleja, A. L. Vázquez de Parga, R. Miranda, J. Camarero, P. Perna

Abstract: The development of graphene (Gr) spintronics requires the ability to engineer epitaxial Gr heterostructures with interfaces of high quality, in which the intrinsic properties of Gr are modified through proximity with a ferromagnet to allow for efficient room temperature spin manipulation or the stabilization of new magnetic textures. These heterostructures can be prepared in a controlled way by in… ▽ More The development of graphene (Gr) spintronics requires the ability to engineer epitaxial Gr heterostructures with interfaces of high quality, in which the intrinsic properties of Gr are modified through proximity with a ferromagnet to allow for efficient room temperature spin manipulation or the stabilization of new magnetic textures. These heterostructures can be prepared in a controlled way by intercalation through graphene of different metals. Using photoelectron spectroscopy (XPS) and Scanning Tunneling Microscopy (STM), we achieve a nanoscale control of thermal activated intercalation of homogeneous ferromagnetic (FM) layer underneath epitaxial Gr grown onto (111)-oriented heavy metal (HM) buffers deposited in turn onto insulating oxide surfaces. XPS and STM demonstrate that Co atoms evaporated on top of Gr arrange in 3D clusters, and, upon thermal annealing, penetrate through and diffuse below Gr in a 2D fashion. The complete intercalation of the metal occurs at specific temperatures depending on the type of metallic buffer. The activation energy and the optimum temperature for the intercalation processes are determined. We describe a reliable method to fabricate and characterize in-situ high quality Gr-FM/HM heterostructures enabling the realization of novel spin-orbitronic devices that exploits the extraordinary properties of Gr. △ Less

Submitted 19 November, 2019; v1 submitted 18 November, 2019; originally announced November 2019.

Comments: 13 pages, 6 figures

Journal ref: ACS Appl. Mater. Interfaces 2019

Tailoring topological order and π-conjugation to engineer quasi-metallic polymers

Authors: B. Cirera, A. Sánchez-Grande, B. de la Torre, J. Santos, S. Edalatmanesh, E. Rodríguez-Sánchez, K. Lauwaet, B. Mallada-Faes, R. Zbořil, R. Miranda, O. Gröning, P. Jelínek, N. Martín, D. Écija

Abstract: Topological band theory provides a conceptual framework to predict or even engineer robust metallic states at the boundaries of topologically distinct phases. The bulk-boundary correspondence requires that a topological electronic phase transition between two insulators must proceed via closing of the electronic gap. Therefore, it can provide a conceptual solution to the instability of metallic ph… ▽ More Topological band theory provides a conceptual framework to predict or even engineer robust metallic states at the boundaries of topologically distinct phases. The bulk-boundary correspondence requires that a topological electronic phase transition between two insulators must proceed via closing of the electronic gap. Therefore, it can provide a conceptual solution to the instability of metallic phases in π-conjugated 1D polymers. In this work we predict and demonstrate that a clever design and on-surface synthesis of polymers consisting of 1D linearly bridged polyacene moieties, can position the resulting polymer near the topological transition from a trivial to a non-trivial quantum phase featuring a very narrow bandgap with in-gap zero-energy edge-states at the topologically non-trivial phase. We also reveal the fundamental connection between topological classes and electronic transformation of 1D π-conjugated polymers. △ Less

Submitted 13 November, 2019; originally announced November 2019.

Unveiling the Radiative Local Density of Optical States of a Plasmonic Nanocavity by STM Luminescence and Spectroscopy

Authors: Alberto Martín-Jiménez, Antonio I. Fernández-Domínguez, Koen Lauwaet, Daniel Granados, Rodolfo Miranda, Francisco J. García-Vidal, Roberto Otero

Abstract: Disentangling the contributions of radiative and non-radiative localized plasmonic modes from the photonic density of states of metallic nanocavities between atomically-sharp tips and flat substrates remains an experimental challenge nowadays. Electroluminescence due to tunnelling through the tip-substrate gap allows discerning solely the excitation of radiative modes, but this information is inhe… ▽ More Disentangling the contributions of radiative and non-radiative localized plasmonic modes from the photonic density of states of metallic nanocavities between atomically-sharp tips and flat substrates remains an experimental challenge nowadays. Electroluminescence due to tunnelling through the tip-substrate gap allows discerning solely the excitation of radiative modes, but this information is inherently convolved with that of the electronic structure of the system. In this work we present a fully experimental procedure to eliminate the electronic-structure factors from the scanning tunnelling microscope luminescence spectra by confronting them with spectroscopic information extracted from elastic current measurements. Comparison against electromagnetic calculations demonstrates that this procedure allows characterizing the meV shifts experienced by the dipolar and quadrupolar plasmonic modes supported by the nanocavity under atomic-scale gap size changes. Our method, thus, gives us access to the frequency-dependent radiative Purcell enhancement that a microscopic light emitter would undergo when placed at the nanocavity. △ Less

Submitted 18 July, 2019; originally announced July 2019.

Robust, switchable, C-related, Superconducting Nanostructure at the Apex of a W tip

Authors: C. G. AYani, F. Calleja, P. Casado, A. Norris, J. J. Navarro, M. Garnica, M. Acebron, D. Granados, A. L. Vazquez de Parga, J. G. Rodrigo, R. Miranda

Abstract: We report on a controlled method to fabricate in-situ a superconducting (SC) nanostructure at the apex of the standard W tip of a Scanning Tunnelling Microscope (STM) by pulsing the tunnelling voltage on a graphene covered metal surface. We have characterized the SC properties of the resulting tip as a function of temperature and magnetic field, obtaining a transition temperature of 3.3 K and a cr… ▽ More We report on a controlled method to fabricate in-situ a superconducting (SC) nanostructure at the apex of the standard W tip of a Scanning Tunnelling Microscope (STM) by pulsing the tunnelling voltage on a graphene covered metal surface. We have characterized the SC properties of the resulting tip as a function of temperature and magnetic field, obtaining a transition temperature of 3.3 K and a critical field well above 3T. The SC nanotip is robust, stable, and achieves atomic resolution. The non-SC tip can be easily recovered by controlled voltage pulsing on a clean metal surface. The present result should be taken into account when studying zero-bias features like Kondo resonances or superconductivity on graphene-based systems by means of STM using tungsten tips. △ Less

Submitted 27 June, 2019; v1 submitted 7 June, 2019; originally announced June 2019.

Engineering Large Anisotropic Magnetoresistance in La0.7Sr0.3MnO3 Films at Room Temperature

Authors: Paolo Perna, Davide Maccariello, Fernando Ajejas, Ruben Guerrero, Laurence Méchin, Stephane Flament, Jacobo Santamaria, Rodolfo Miranda, Julio Camarero

Abstract: The magnetoresistance (MR) effect is widely employed in technologies that pervade our world from magnetic reading heads to sensors. Diverse contributions to MR, such as anisotropic, giant, tunnel, colossal, and spin-Hall, are revealed in materials depending on the specific system and measuring configuration. Half-metallic manganites hold promise for spintronic applications but the complexity of co… ▽ More The magnetoresistance (MR) effect is widely employed in technologies that pervade our world from magnetic reading heads to sensors. Diverse contributions to MR, such as anisotropic, giant, tunnel, colossal, and spin-Hall, are revealed in materials depending on the specific system and measuring configuration. Half-metallic manganites hold promise for spintronic applications but the complexity of competing interactions has not permitted the understanding and control of their magnetotransport properties to enable the realization of their technological potential. Here we report on the ability to induce a dominant switchable magnetoresistance in La0.7Sr0.3MnO3 epitaxial films, at room temperature (RT). By engineering an extrinsic magnetic anisotropy, we show a large enhancement of anisotropic magnetoresistance (AMR) which leads to, at RT, signal changes much larger than the other contributions such as the colossal magnetoresistance (CMR). The dominant extrinsic AMR exhibits large variation in the resistance in low field region, showing high sensitivity to applied low magnetic fields. These findings have a strong impact on the real applications of manganite based devices for the high-resolution low field magnetic sensors or spintronics. △ Less

Submitted 26 November, 2018; originally announced November 2018.

Comments: 10 pages, 6 figures

Journal ref: Adv. Funct. Mater. 2017, 27, 1700664

Discrete Electronic Sub-bands without Confinement: Bragg Scattering at Molecular Edges

Authors: Alberto Martín-Jiménez, José María Gallego, Rodolfo Miranda, Roberto Otero

Abstract: The discretization of the electronic structure of nanometer-size solid systems due to quantum confinement and the concomitant modification of their physical properties is one of the cornerstones for the development of Nanoscience and Nanotechnology. In this letter we demonstrate that Bragg scattering of Cu(111) surface state electrons by the periodic arrangement of tetracyanoquinodimethane (TCNQ)… ▽ More The discretization of the electronic structure of nanometer-size solid systems due to quantum confinement and the concomitant modification of their physical properties is one of the cornerstones for the development of Nanoscience and Nanotechnology. In this letter we demonstrate that Bragg scattering of Cu(111) surface state electrons by the periodic arrangement of tetracyanoquinodimethane (TCNQ) molecules at the edges of self-assembled molecular islands, discretizes the possible values of the electron momentum parallel to the island edge. The electronic structure consists thus of a discrete number of sub-bands which occur in a non-closed space and, therefore, without quantum confinement. △ Less

Submitted 5 October, 2018; v1 submitted 26 July, 2018; originally announced July 2018.

Journal ref: Phys. Rev. Lett. 122, 176801 (2019)

Ethanol chemisorption on core-shell Pt-nanoparticles: an ab-initio study

Authors: Vagner A. Rigo, Caetano R. Miranda, Francesca Baletto

Abstract: By means of ab-initio calculations, we have investigated the chemisorption paroperties of ethanol onto segregating binary nanoalloys. We select nanostructures with icosahedral shape of 55 atoms with a Pt outermost layer over a M core with M=Ag,Pd,Ni. With respect to nanofilms with equivalent composition, there is an increse of the ethanol binding energy. This is not merely due to observed shorteni… ▽ More By means of ab-initio calculations, we have investigated the chemisorption paroperties of ethanol onto segregating binary nanoalloys. We select nanostructures with icosahedral shape of 55 atoms with a Pt outermost layer over a M core with M=Ag,Pd,Ni. With respect to nanofilms with equivalent composition, there is an increse of the ethanol binding energy. This is not merely due to observed shortening of the Pt-O distance but depends on the nanoparticle distortion after ethanol adsorption. This geometrical distortion within the nanoparticle can be interpreted as a radial breathing, which is sensitive to the adsortion site, identified by the O-anchor point and the relative positions of the ethyl group. More interestingly, being core-dependent -larger in Pd@Pt and smaller in Ni@Pt-, it relates to an effective electron transfer from ethanol and the M-core towards the Pt-shell. On the view of this new analysis, Pd@Pt nanoalloys show the most promissing features for ethanol oxidation. △ Less

Submitted 20 April, 2018; originally announced April 2018.

Unravelling Dzyaloshinskii-Moriya interaction and chiral nature of Graphene/Cobalt interface

Authors: Fernando Ajejas, Adrian Gudín, Ruben Guerrero, Miguel Angel Niño, Stefania Pizzini, Jan Vogel, Manuel Valvidares, Pierluigi Gargiani, Mariona Cabero, Maria Varela, Julio Camarero, Rodolfo Miranda, Paolo Perna

Abstract: A major challenge for future spintronics is to develop suitable spin transport channels with long spin lifetime and propagation length. Graphene can meet these requirements, even at room temperature. On the other side, taking advantage of the fast motion of chiral textures, i.e., Néel-type domain walls and magnetic skyrmions, can satisfy the demands for high-density data storage, low power consump… ▽ More A major challenge for future spintronics is to develop suitable spin transport channels with long spin lifetime and propagation length. Graphene can meet these requirements, even at room temperature. On the other side, taking advantage of the fast motion of chiral textures, i.e., Néel-type domain walls and magnetic skyrmions, can satisfy the demands for high-density data storage, low power consumption and high processing speed. We have engineered epitaxial structures where an epitaxial ferromagnetic Co layer is sandwiched between an epitaxial Pt(111) buffer grown in turn onto MgO(111) substrates and a graphene layer. We provide evidence of a graphene-induced enhancement of the perpendicular magnetic anisotropy up to 4 nm thick Co films, and of the existence of chiral left-handed Néel-type domain walls stabilized by the effective Dzyaloshinskii-Moriya interaction (DMI) in the stack. The experiments show evidence of a sizeable DMI at the gr/Co interface, which is described in terms of a conduction electron mediated Rashba-DMI mechanism and points opposite to the Spin Orbit Coupling-induced DMI at the Co/Pt interface. In addition, the presence of graphene results in: i) a surfactant action for the Co growth, producing an intercalated, flat, highly perfect fcc film, pseudomorphic with Pt and ii) an efficient protection from oxidation. The magnetic chiral texture is stable at room temperature and grown on insulating substrate. Our findings open new routes to control chiral spin structures using interfacial engineering in graphene-based systems for future spin-orbitronics devices fully integrated on oxide substrates. △ Less

Submitted 2 April, 2018; v1 submitted 20 March, 2018; originally announced March 2018.

Comments: 10 pages, 6 figures

Journal ref: Nano Letters 2018

Interfacial exchange coupling induced chiral symmetry-breaking of Spin-Orbit effects

Authors: P. Perna, F. Ajejas, D. Maccariello, J. L. Fernandez Cuñado, R. Guerrero, M. A. Niño, A. Bollero, R. Miranda, J. Camarero

Abstract: We demonstrate that the interfacial exchange coupling in ferromagnetic/antiferromagnetic (FM/AFM) systems induces symmetry-breaking of the Spin-Orbit (SO) effects. This has been done by studying the field and angle dependencies of anisotropic magnetoresistance and vectorialresolved magnetization hysteresis loops, measured simultaneously and reproduced with numerical simulations. We show how the in… ▽ More We demonstrate that the interfacial exchange coupling in ferromagnetic/antiferromagnetic (FM/AFM) systems induces symmetry-breaking of the Spin-Orbit (SO) effects. This has been done by studying the field and angle dependencies of anisotropic magnetoresistance and vectorialresolved magnetization hysteresis loops, measured simultaneously and reproduced with numerical simulations. We show how the induced unidirectional magnetic anisotropy at the FM/AFM interface results in strong asymmetric transport behaviors, which are chiral around the magnetization hard-axis direction. Similar asymmetric features are anticipated in other SO-driven phenomena. △ Less

Submitted 23 December, 2017; originally announced December 2017.

Comments: 6 pages, 5 figures

Journal ref: Physical Review B 92, 220422(R) (2015)

Periodic spatial variation of the electron-phonon interaction in epitaxial graphene on Ru(0001

Authors: Andres Castellanos-Gomez, Gabino Rubio-Bollinger, Sara Barja, Manuela Garnica, Amadeo L. Vázquez de Parga, Rodolfo Miranda, Nicolás Agraït

Abstract: We have performed low temperature scanning tunnelling spectroscopy (STS) measurements on graphene epitaxially grown on Ru(0001). An inelastic feature, related to the excitation of a vibrational breathing mode of the graphene lattice, was found at 360 meV. The change in the differential electrical conductance produced by this inelastic feature, which is associated with the electron-phonon interacti… ▽ More We have performed low temperature scanning tunnelling spectroscopy (STS) measurements on graphene epitaxially grown on Ru(0001). An inelastic feature, related to the excitation of a vibrational breathing mode of the graphene lattice, was found at 360 meV. The change in the differential electrical conductance produced by this inelastic feature, which is associated with the electron-phonon interaction strength, varies spatially from one position to other of the graphene supercell. This inhomogeneity in the electronic properties of graphene on Ru(0001) results from local variations of the carbon-ruthenium interaction due to the lattice mismatch between the graphene and the Ru(0001) lattices. △ Less

Submitted 22 February, 2013; originally announced February 2013.

Comments: 6 Pages, 3 figures

Journal ref: Appl. Phys. Lett. 102, 063114 (2013

Highly reproducible low temperature scanning tunnelling microscopy and spectroscopy with in situ prepared tips

Authors: Andres Castellanos-Gomez, Gabino Rubio-Bollinger, Manuela Garnica, Sara Barja, Amadeo L. Vázquez de Parga, Rodolfo Miranda, Nicolás Agraït

Abstract: An in situ tip preparation procedure compatible with ultra-low temperature and high magnetic field scanning tunneling microscopes is presented. This procedure does not require additional preparation techniques such as thermal annealing or ion milling. It relies on the local electric-field-induced deposition of material from the tip onto the studied surface. Subsequently, repeated indentations are… ▽ More An in situ tip preparation procedure compatible with ultra-low temperature and high magnetic field scanning tunneling microscopes is presented. This procedure does not require additional preparation techniques such as thermal annealing or ion milling. It relies on the local electric-field-induced deposition of material from the tip onto the studied surface. Subsequently, repeated indentations are performed onto the sputtered cluster to mechanically anneal the tip apex and thus to ensure the stability of the tip. The efficiency of this method is confirmed by comparing the topography and spectroscopy data acquired with either unprepared or in situ prepared tips on epitaxial graphene grown on Ru (0001). We demonstrate that the use of in situ prepared tips increases the stability of the scanning tunneling images and the reproducibility of the spectroscopic measurements. △ Less

Submitted 4 September, 2012; originally announced September 2012.

Comments: 9 Pages, 4 figures (including supp. material)

Journal ref: Ultramicroscopy 122 (2012) 1-5

Electronic and Geometric Corrugation of Periodically Rippled, Self-nanostructured Graphene Epitaxially Grown on Ru(0001)

Authors: Bogdana Borca, Sara Barja, Manuela Garnica, Marina Minniti, Antonio Politano, Josefa M. Rodriguez-García, Juan Jose Hinarejos, Daniel Farías, Amadeo L. Vázquez de Parga, Rodolfo Miranda

Abstract: Graphene epitaxially grown on Ru(0001) displays a remarkably ordered pattern of hills and valleys in Scanning Tunneling Microscopy (STM) images. To which extent the observed "ripples" are structural or electronic in origin have been much disputed recently. A combination of ultrahigh resolution STM images and Helium Atom diffraction data shows that i) the graphene lattice is rotated with respect to… ▽ More Graphene epitaxially grown on Ru(0001) displays a remarkably ordered pattern of hills and valleys in Scanning Tunneling Microscopy (STM) images. To which extent the observed "ripples" are structural or electronic in origin have been much disputed recently. A combination of ultrahigh resolution STM images and Helium Atom diffraction data shows that i) the graphene lattice is rotated with respect to the lattice of Ru and ii) the structural corrugation as determined from He diffraction is substantially smaller (0.015 nm) than predicted (0.15 nm) or reported from X-Ray Diffraction or Low Energy Electron Diffraction. The electronic corrugation, on the contrary, is strong enough to invert the contrast between hills and valleys above +2.6 V as new, spatially localized electronic states enter the energy window of the STM. The large electronic corrugation results in a nanostructured periodic landscape of electron and holes pockets. △ Less

Submitted 11 August, 2010; v1 submitted 11 May, 2010; originally announced May 2010.

Comments: 16 pages, 6 figures

Journal ref: New Journal of Physics 12, 093018 (2010)

Tailoring magnetic anisotropy in epitaxial half metallic La0.7Sr0.3MnO3 thin films

Authors: P. Perna, C. Rodrigo, E. Jiménez, F. J. Teran, L. Méchin, N. Mikuszeit, J. Camarero, R. Miranda

Abstract: We present a detailed study on the magnetic properties, including anisotropy, reversal fields, and magnetization reversal processes, of well characterized half-metallic epitaxial La0.7Sr0.3MnO3 (LSMO) thin films grown onto SrTiO3 (STO) substrates with three different surface orientations, i.e. (001), (110) and (1-18). The latter shows step edges oriented parallel to the [110] (in-plane) crystallog… ▽ More We present a detailed study on the magnetic properties, including anisotropy, reversal fields, and magnetization reversal processes, of well characterized half-metallic epitaxial La0.7Sr0.3MnO3 (LSMO) thin films grown onto SrTiO3 (STO) substrates with three different surface orientations, i.e. (001), (110) and (1-18). The latter shows step edges oriented parallel to the [110] (in-plane) crystallographic direction. Room temperature high resolution vectorial Kerr magnetometry measurements have been performed at different applied magnetic field directions in the whole angular range. In general, the magnetic properties of the LSMO films can be interpreted with just the uniaxial term with the anisotropy axis given by the film morphology, whereas the strength of this anisotropy depends on both structure and film thickness. In particular, LSMO films grown on nominally flat (110)-oriented STO substrates presents a well defined uniaxial anisotropy originated from the existence of elongated in-plane [001]-oriented structures, whereas LSMO films grown on nominally flat (001)-oriented STO substrates show a weak uniaxial magnetic anisotropy with the easy axis direction aligned parallel to residual substrate step edges. Elongated structures are also found for LSMO films grown on vicinal STO(001) substrates. These films present a well-defined uniaxial magnetic anisotropy with the easy axis lying along the step edges and its strength increases with the LSMO thickness. It is remarkable that this step-induced uniaxial anisotropy has been found for LSMO films up to 120 nm thickness. Our results are promising for engineering novel half-metallic magnetic devices that exploit tailored magnetic anisotropy. △ Less

Submitted 9 March, 2011; v1 submitted 3 May, 2010; originally announced May 2010.

Comments: 10 pages, 10 figures, 1 table

Journal ref: JOURNAL OF APPLIED PHYSICS 110, 013919 (2011)

Potential Energy Landscape for hot electrons in periodically nanostructured graphene

Authors: B. Borca, S. Barja, M. Garnica, D. Sanchez-Portal, V. M. Silkin, E. V. Chulkov, F. Hermanns, J. J. Hinarejos, A. L. Vazquez de Parga, A. Arnau, P. M. Echenique, R. Miranda

Abstract: We explore the spatial variations of the unoccupied electronic states of graphene epitaxially grown on Ru(0001) and observed three unexpected features: the first graphene image state is split in energy, unlike all other image states, the split state does not follow the local work function modulation, and a new interfacial state at +3 eV appears on some areas of the surface. These results show the… ▽ More We explore the spatial variations of the unoccupied electronic states of graphene epitaxially grown on Ru(0001) and observed three unexpected features: the first graphene image state is split in energy, unlike all other image states, the split state does not follow the local work function modulation, and a new interfacial state at +3 eV appears on some areas of the surface. These results show the system behaves as a self-organized periodic array of quantum dots. △ Less

Submitted 5 April, 2010; originally announced April 2010.

Comments: 20 pages, 6 figures

Journal ref: Physical Review Letters 105, 036804 (2010)

Periodically modulated geometric and electronic structure of graphene on Ru(0001)

Authors: B. Borca, S. Barja, M. Garnica, J. J. Hinarejos, A. L. Vazquez de Parga, R. Miranda, F. Guinea

Abstract: We report here on a method to fabricate and characterize highly perfect, periodically rippled graphene monolayers and islands, epitaxially grown on single crystal metallic substrates under controlled UHV conditions. The periodicity of the ripples is dictated by the difference in lattice parameters of graphene and substrate, and, thus, it is adjustable. We characterize its perfection at the atomi… ▽ More We report here on a method to fabricate and characterize highly perfect, periodically rippled graphene monolayers and islands, epitaxially grown on single crystal metallic substrates under controlled UHV conditions. The periodicity of the ripples is dictated by the difference in lattice parameters of graphene and substrate, and, thus, it is adjustable. We characterize its perfection at the atomic scale by means of STM and determine its electronic structure in the real space by local tunnelling spectroscopy. There are periodic variations in the geometric and electronic structure of the graphene monolayer. We observe inhomogeneities in the charge distribution, i.e a larger occupied Density Of States at the higher parts of the ripples. Periodically rippled graphene might represent the physical realization of an ordered array of coupled graphene quantum dots. The data show, however, that for rippled graphene on Ru(0001) both the low and the high parts of the ripples are metallic. The fabrication of periodically rippled graphene layers with controllable characteristic length and different bonding interactions with the substrate will allow a systematic experimental test of this fundamental problem. △ Less

Submitted 27 March, 2009; originally announced March 2009.

Comments: 12 pages. Contribution to the topical issue on graphene of Semiconductor Science and Technology

Periodically rippled graphene: growth and spatially resolved electronic structure

Authors: A. L. Vazquez de Parga, F. Calleja, B. Borca, M. C. G. Passeggi Jr, J. J. Hinarejo, F. Guinea, R. Miranda

Abstract: We studied the growth of an epitaxial graphene monolayer on Ru(0001). The graphene monolayer covers uniformly the Ru substrate over lateral distances larger than several microns reproducing the structural defects of the Ru substrate. The graphene is rippled with a periodicity dictated by the difference in lattice parameter between C and Ru. The theoretical model predict inhomogeneities in the el… ▽ More We studied the growth of an epitaxial graphene monolayer on Ru(0001). The graphene monolayer covers uniformly the Ru substrate over lateral distances larger than several microns reproducing the structural defects of the Ru substrate. The graphene is rippled with a periodicity dictated by the difference in lattice parameter between C and Ru. The theoretical model predict inhomogeneities in the electronic structure. This is confirmed by measurements in real space by means of scanning tunnelling spectroscopy. We observe electron pockets at the higher parts of the ripples. △ Less

Submitted 4 September, 2007; originally announced September 2007.

Comments: 5 pages

Journal ref: Phys. Rev. Lett., vol. 100, 056807 (2008)

A stacking-fault based microscopic model for platelets in diamond

Authors: C. R. Miranda, R. W. Nunes, A. Antonelli

Abstract: We propose a new microscopic model for the $\{001\}$ planar defects in diamond commonly called platelets. This model is based on the formation of a metastable stacking fault, which can occur because of the ability of carbon to stabilize in different bonding configurations. In our model the core of the planar defect is basically a double layer of three-fold coordinated $sp^2$ carbon atoms embedde… ▽ More We propose a new microscopic model for the $\{001\}$ planar defects in diamond commonly called platelets. This model is based on the formation of a metastable stacking fault, which can occur because of the ability of carbon to stabilize in different bonding configurations. In our model the core of the planar defect is basically a double layer of three-fold coordinated $sp^2$ carbon atoms embedded in the common $sp^3$ diamond structure. The properties of the model were determined using {\it ab initio} total energy calculations. All significant experimental signatures attributed to the platelets, namely, the lattice displacement along the $[001]$ direction, the asymmetry between the $[110]$ and the $[1\bar{1}0]$ directions, the infrared absorption peak $B^\prime$, and broad luminescence lines that indicate the introduction of levels in the band gap, are naturally accounted for in our model. The model is also very appealing from the point of view of kinetics, since naturally occurring shearing processes will lead to the formation of the metastable fault. △ Less

Submitted 20 September, 2004; originally announced September 2004.

Comments: 5 pages, 4 figures. Submitted for publication on August 2nd, 2004

Temperature effects on dislocation core energies in silicon and germanium

Authors: Caetano R. Miranda, Ricardo W. Nunes, A. Antonelli

Abstract: Temperature effects on the energetics of the 90-degree partial dislocation in silicon and germanium are investigated, using non-equilibrium methods to estimate free energies, coupled with Monte Carlo simulations. Atomic interactions are described by Tersoff and EDIP interatomic potentials. Our results indicate that the vibrational entropy has the effect of increasing the difference in free energ… ▽ More Temperature effects on the energetics of the 90-degree partial dislocation in silicon and germanium are investigated, using non-equilibrium methods to estimate free energies, coupled with Monte Carlo simulations. Atomic interactions are described by Tersoff and EDIP interatomic potentials. Our results indicate that the vibrational entropy has the effect of increasing the difference in free energy between the two possible reconstructions of the 90-degree partial, namely, the single-period and the double-period geometries. This effect further increases the energetic stability of the double-period reconstruction at high temperatures. The results also indicate that anharmonic effects may play an important role in determining the structural properties of these defects in the high-temperature regime. △ Less

Submitted 20 September, 2004; originally announced September 2004.

Comments: 8 pages in two-column physical-review format with six figures

Journal ref: Physical Review B vol 67, paper# 235201, 2003

Non-stochastic behavior of atomic surface diffusion on Cu(111) at all temperatures

Authors: J. Ferrón, L. Gómez, J. J. de Miguel, R. Miranda

Abstract: Atomic diffusion is usually understood as a succession of random, independent displacements of an adatom over the surface's potential energy landscape. Nevertheless, an analysis of Molecular Dynamics simulations of self-diffusion on Cu(111) demonstrates the existence of different types of correlations in the atomic jumps at all temperatures. Thus, the atomic displacements cannot be correctly des… ▽ More Atomic diffusion is usually understood as a succession of random, independent displacements of an adatom over the surface's potential energy landscape. Nevertheless, an analysis of Molecular Dynamics simulations of self-diffusion on Cu(111) demonstrates the existence of different types of correlations in the atomic jumps at all temperatures. Thus, the atomic displacements cannot be correctly described in terms of a random walk model. This fact has a profound impact on the determination and interpretation of diffusion coefficients. △ Less

Submitted 22 March, 2004; originally announced March 2004.

Comments: 5 figures

Detecting Electronic States at Stacking Faults in Magnetic Thin Films by Tunneling Spectroscopy

Authors: A. L. Vazquez de Parga, F. J. Garcia-Vidal, R. Miranda

Abstract: Co islands grown on Cu(111) with a stacking fault at the interface present a conductance in the empty electronic states larger than the Co islands that follow the stacking sequence of the Cu substrate. Electrons can be more easily injected into these faulted interfaces, providing a way to enhance transmission in future spintronic devices. The electronic states associated to the stacking fault ar… ▽ More Co islands grown on Cu(111) with a stacking fault at the interface present a conductance in the empty electronic states larger than the Co islands that follow the stacking sequence of the Cu substrate. Electrons can be more easily injected into these faulted interfaces, providing a way to enhance transmission in future spintronic devices. The electronic states associated to the stacking fault are visualized by tunneling spectroscopy and its origin is identified by band structure calculations. △ Less

Submitted 28 September, 2000; originally announced September 2000.

Comments: 4 pages, 4 figures; to be published in Phys. Rev. Lett (2000)